PRACTICAL SKILLS AND TECHNIQUES Flashcards
Describe the relationship that colorimetry uses to determine concentration.
Colorimetry uses the relationship between the color intensity of a solution and the concentration of the colored species present.
Explain the purpose of a colorimeter or spectrophotometer in colorimetry.
used to measure the absorbance of light of a series of standard solutions, which is then used to plot a calibration graph.
How is the concentration of a solution determined using colorimetry?
The concentration of the solution being tested is determined from its absorbance by referring to the calibration curve.
What is the significance of the straight line section of the calibration graph in colorimetry?
The concentration of colored species in the solution being tested must lie in the straight line section of the calibration graph for accurate determination.
Define distillation and its primary uses in chemistry.
Distillation is a technique used for the identification and purification of organic compounds.
How can the boiling point of a compound assist in confirming its identity?
The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm its identity.
Explain how distillation can purify a compound.
seperates it from less volatile substances in the mixture.
Describe the purpose of heating under reflux in chemical reactions.
Allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
Explain the setup required for heating under reflux.
The reaction mixture is placed in a round-bottomed flask with anti-bumping granules, and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat.
Define vacuum filtration and its purpose in chemistry.
carries out filtration under reduced pressure
provides a faster means of separating a precipitate from a filtrate.
Identify the types of funnels that can be used in vacuum filtration.
A Büchner funnel, Hirsch funnel, or sintered glass funnel can be used during vacuum filtration.
How does heating under reflux prevent the loss of volatile substances?
allows them to condense and return to the reaction mixture instead of escaping.
What are anti-bumping granules and their role in reflux heating?
Anti-bumping granules are small particles added to the reaction mixture to promote smooth boiling and prevent bumping during heating.
Discuss the advantages of using vacuum filtration over regular filtration.
Vacuum filtration is faster than regular filtration because it uses reduced pressure to expedite the separation of solids from liquids.
How is a condenser used in the heating under reflux process?
cool and condense vapors back into the liquid phase, allowing them to return to the reaction mixture.
Describe the process of recrystallisation for purifying an impure solid.
dissolve an impure solid in a minimum volume of hot solvent to remove insoluble impurities
cool the filtrate slowly to allow pure crystals to form,
filtering, washing, and drying the pure crystals.
Explain the criteria for choosing a solvent in recrystallisation.
The solvent should allow the compound to be completely soluble at high temperatures and only sparingly soluble at lower temperatures.
Do you know the steps involved in hot filtration during recrystallisation?
Hot filtration involves filtering the hot solution to remove any insoluble impurities before allowing the solution to cool and form crystals.
How does cooling the filtrate contribute to the recrystallisation process?
Cooling the filtrate slowly allows crystals of the pure compound to form while leaving soluble impurities dissolved in the solvent.
Define solvent extraction and its purpose.
isolates solute from a liquid mixture or solution by using an immiscible solvent in which the solute is soluble.
Explain the role of the separating funnel in solvent extraction.
The separating funnel allows the two immiscible solvents to form two layers, enabling the solute to dissolve in both and establish an equilibrium between the layers.
How is the equilibrium constant, K, related to solvent extraction?
The equilibrium constant, K, determines the ratio of solute dissolved in each layer of the immiscible solvents during solvent extraction.
Describe the procedure for maximizing solute extraction in solvent extraction.
Lower layer is run off into one container, and the upper layer is poured into a second container. This process is repeated.
What happens to the solute during solvent extraction?
The solute dissolves in both immiscible solvents, allowing for its separation from the liquid mixture.
Describe the effect of multiple extractions with smaller volumes of solvent on solute extraction.
The quantity of solute extracted is greater when multiple extractions using smaller volumes of solvent are performed
Define the characteristics of an ideal solvent for extraction processes.
immiscible with the liquid mixture,
have higher solubility for the solute than the liquid mixture,
be volatile for easy evaporation,
be unreactive with the solute.
Explain the significance of melting point determination in confirming the identity of a compound.
Melting point determination is significant because it allows for the comparison of the experimentally determined melting point with known literature values, confirming the identity of a pure compound.
How does the presence of impurities affect the melting point of a compound?
The presence of impurities lowers the melting point and broadens the melting temperature range due to disruption in intermolecular bonding within the crystal lattice.
Do multiple extractions improve the efficiency of solute recovery?
Yes, multiple extractions improve the efficiency of solute recovery compared to a single extraction with a larger volume of solvent.
Explain the melting point range of a substance.
The melting point range of a substance is the temperature range from when the solid first begins to melt to when all of the solid has completely melted.
Describe how melting point analysis can indicate the purity of a compound.
Melting point analysis can indicate purity because a pure compound has a sharp melting point, while impurities cause a lower and broader melting point range.
What are the requirements for a solvent to be effective in extracting a solute from a liquid mixture?
immiscible with the liquid mixture,
more soluble for the solute,
volatile for evaporation,
unreactive with the solute.
Describe the process of determining a mixed melting point.
A mixed melting point is determined by mixing a small quantity of the product with some of the pure compound and measuring the melting point. The melting point value and range help to ascertain if the product and the pure compound are the same substance.
Explain the purpose of chromatography in separating mixtures.
Chromatography is used to separate the components of a mixture based on differences in their polarity or molecular size.
Define thin-layer chromatography (TLC).
Thin-layer chromatography (TLC) is a technique that uses a fine film of silica or aluminium oxide spread over a substrate to separate components of a mixture.
How is a sample applied in thin-layer chromatography?
A small sample of the mixture is spotted onto the base (pencil) line of the chromatogram.
What role does the solvent play in thin-layer chromatography?
The solvent dissolves the compounds in the spot and carries them up the chromatogram, with the distance traveled depending on the solubility of the compounds and their adherence to the plate.
Describe how the visualization of spots on a chromatogram is achieved.
Visualization of spots on a chromatogram is typically achieved using a developing agent or ultraviolet light.
What factors influence how far compounds travel in thin-layer chromatography?
The distance compounds travel in TLC is influenced by their solubility in the solvent and how well they adhere to the plate.
Explain the significance of melting point determination in identifying substances.
Determining the melting point helps to confirm whether a product and a pure compound are the same substance based on their melting point value and range.
How are Rf values calculated
distance travelled by sample / distance travelled by solvent
How can the identity of a compound be confirmed
comparing the experimentally determined f
R values with a literature or known value
determined under the same conditions
making a direct comparison on a TLC plate between the compound being tested and
the pure substance — a co-spot could be used
What do multiple spots in TLC suggest
sample is impure
